The present invention relates to an apparatus for obtaining image information from a living body under examination through use of a nuclear magnetic resonance signal, and more particularly to the relative arrangement of its static magnetic field and radio-frequency magnetic field and the examinee's body.
Many literatures have already been published on apparatus for obtaining image information from a living body particularly, human body through utilization of a nuclear magnetic resonance (hereinafter referred to simply as NMR) signal (which apparatus will hereinafter be referred to as the NMR-CT apparatus).
To begin with, the NMR phenomenon occurs through such a mechanism as follows: When placed in a static magnetic field, atomic nuclei resonate with a component of a radio-frequency magnetic field (hereinafter referred to as RF magnetic field or simply as RF field) of a specified frequency proportional to the intensity of the static magnetic field (which may hereinafter be referred to as static field which is perpendicular to that component, and precess about an axis in the direction of application of the static magnetic field at the above frequency (a resonance frequency). By the precession the atomic nuclei absorb the energy of the RF magnetic field and are excited and upon completion of the excitation, they relax while releasing, as an NMR signal, a portion of the absorbed energy of the RF field. The resonance frequency is known under the name of a Larmor frequency and is given by .omega..sub.e =.gamma..multidot.H.sub.e, where .gamma. is a nuclear gyromagnetic ratio and H.sub.e is the intensity of the static magnetic field.
Then, when a magnetic field which has the same direction as the static magnetic field and whose intensity varies along a specified direction, that is, a so-called gradient magnetic field (which may hereinafter be referred to as gradient field), is superimposed on the spatially homogeneous static magnetic field, atomic nuclei (hereinafter referred to as nuclear spins) at respective coordinates in the above specified direction precess at different frequencies by virtue of the RF magnetic field.
It can be said that it is the NMR-CT that obtains, by an ingenious utilization of abovementioned property, the spatial distribution of information (the nuclear spin density, the relaxation time, etc.) contained in the NMR signal. FIG. 1 illustrates in block form specific constituents of the NMR-CT apparatus. A static magnetic field generating magnet 11 applies a static magnetic field to a living body (not shown) and a gradient magnetic field generating coil 12 applies thereto a gradient magnetic field. The gradient field is identical in direction with the static field and its intensity is graded in three directions which usually intersect one another at right angles. This field arrangement permits discrimination of spatial information of the body. RF power in the resonance frequency band is provided from a transmitter 13 via an automatic transmission/reception switching circuit 14 to an RF coil 15. An RF magnetic field, which is perpendicular to the static field, is applied from the RF coil 15 to the body. The resulting NMR signal from the body is received by the RF coil 15, from which it is supplied to a receiver 17 via the automatic transmission/reception switching circuit 14 and a receiving coil select circuit 16. After being amplified and detected in the receiver 17 the NMR signal is converted by an A/D converter 18 into a digital signal. The digital signal is provided to a computer 19, in which it is subjected to a calculation process for image reconstruction, etc., and its results are displayed as an image on a display 20. The static field generating magnet 11 is excited by a static field generating means 11a and the gradient field generating coil 12 by a gradient field generating means 12a.
When the direction of the magnetic fields emanating from the static field generating magnet 11 and the gradient field generating coil 12 (which direction will hereinafter be referred to as the static field direction) perpendicularly intersect the direction of the RF magnetic field (hereinafter referred to as the RF direction) from the RF coil 15, the application of the RF field and the detection of the NMR signal can be achieved with the highest efficiency. The NMR-CT apparatus satisfying above conditions are roughly divided into two groups in terms of the magnetic field generating structure. In a group 1 the static field direction and the direction in which a human body is taken into and out of the static field are perpendicular to each other, whereas in a group 2 the both directions are parallel to each other.
Since it is necessary that the static field direction and the RF direction be perpendicular to each other, as referred to above, a solenoid coil is often used as the RF coil in the case of the group 1, and in the case of the group 2 a saddle-shaped coil is usually employed.
The RF coil for detecting the NMR signal is naturally one of important components which determine the SN ratio of the NMR-CT. The solenoid coil is about three times higher than the saddle-shaped coil in RF sensitivity. Recently there has been a strong demand for higher sensitivity, and an RF coil commonly referred to as a surface coil is often used according to the region of examination (limited to regions near the skin surface). Usually the surface coil has a spiral configuration, as shown in FIG. 2A, which is substantially flat and small in the number of turns. The RF field emanating from the surface coil is perpendicular to the coil plane. The surface coil is designed so that the SN ratio of the NMR signal from the region of examination may be maximum taking into account an area of the region of examination. In practical use the surface coil is applied to the skin surface corresponding to the region of examination with the coil plane held in parallel to the skin surface.
As depicted in FIG. 1, a surface coil 21 is connected to the receiver 17 via the receiving coil select circuit 16 which switches between the surface coil 21 and the solenoid coil 15 as required. That is, the RF field is applied to the body from the solenoid coil 15 and the NMR signal from the body is received by the solenoid coil 15 and the surface coil 21, and selectively provided via the receiving coil select circuit 16 to the receiver 17. The receiving coil select circuit 16 selectively connects the automatic transmission/reception switching circuit 14 and the surface coil 21 to the receiver 17. It is possible to arrange such that the solenoid coil 15 and the surface coil 21 are selectively connected to the automatic transmission/reception switching circuit 14, the receiver 17 is connected directly to the automatic transmission/reception switching circuit 14. In this case, if desired, the surface coil 21 may be used both for generation of the RF field and reception of NMR signal.
Chief regions of examination through use of the surface coil are, in the case of the human body, an eye, the breast, the backbone, the heart, the liver, the kidney and so forth. In view of the facts that the human body has an elliptic cross-section in a transverse direction in which the breadth of the body is larger than the thickness of the body, that vertical dimensions of the body are far larger than the dimensions of the transverse cross-section and that the examinee usually lies on his back during examination, it is highly desirable that the surface coil be disposed with its RF direction held vertical.
In this instance, the static field direction must be made horizontal so as to meet the aforementioned requirement that the static field direction and the RF direction be perpendicular to each other.
In the magnet system of the group 2 the static field direction is parallel to the longitudinal axis of the lying human body, and hence is originally horizontal, but in the magnet system of the group 1 the static field direction is conventionally limited specifically to the direction perpendicular to the lying human body. In the group 1 the static field generating means employing permanent magnets is such as shown in FIG. 3 in which different magnetic poles of the permanent magnets 22 and 23 are disposed opposite but spaced apart in the vertical direction and are interconnected via a magnetic yoke 24, setting up a vertical static field 25 between the opposing faces of the magnets 22 and 23. A solenoid coil 26 for the RF magnetic field is disposed in the static magnetic field 25. The axis of the solenoid coil 26 is extended in a horizontal direction and is perpendicular to the static field 25. In the solenoid coil 26 a deck 27 is horizontally disposed in a manner to be slidable in parallel with the axis of the solenoid coil 26 and an examinee 28 lies on the deck 27.
Since the static field direction is vertical, as mentioned above, the magnet system of the group 1 presents the problem that the surface coil cannot be used with its RF direction held vertical.